1. Field of the Invention
This invention relates in general to radio receivers and in particular to radio receivers with audio enhancing capabilities.
2. Description of the Related Art
Recent requirements by the Federal Communications Commission (FCC) have mandated communication device suppliers to design receivers for use in communication devices with highly restrictive channel bandwidths (i.e. 12.5 KHz, 6.25 KHz). This restriction on the bandwidth of a signal inherently degrades the audio performance of the communication device. The degradation can be attributed to the narrower intermediate frequency (IF) bandwidths necessary to obtain adjacent channel and other receiver performance properties.
For a given set of specifications there is a maximum intermediate frequency bandwidth that can be used on the receiver to satisfy these specifications. Detrimentally, as the channel spacing decreases (i.e. 12.5 KHz, 6.25 KHz), the intermediate frequency bandwidths needed for operating within the associated radio frequency (RF) characteristics of the communication link begin to attenuate the high frequency audio components of the received signal. Audio signal clarity and intelligibility has been shown to be highly correlated to the high frequency content of the signal. This attenuation of the high frequency components results in perceived degradation in the crispness and punch through of the demodulated audio signal. The communication device performance, while benefiting from having higher channels for a given spectrum, is compromised by the degradation on the intelligibility of the recovered audio.
In conventional designs, the intermediate frequency bandwidth (IFBW) of a receiver is primarily determined by the channel spacing of the receiver combined with the specifications the receiver is designed to meet (i.e. adjacent channel, sensitivity, etc). As channel spacing is reduced, the intermediate frequency bandwidth must be reduced in order to maintain the same RF specifications of the receiver. Eventually a point is reached where the IFBW is too small and the demodulated receive audio suffers in intelligibility and crispness. FIG. 1 illustrates the recovered audio spectrum of a typical receiver with the IFBW set for a typical narrow channel spacing system. As illustrated, the components on the higher end of the audio spectrum (2.5 Khz-3.5 KHz) are considerably attenuated.
Therefore, what is needed is a means for providing superior audio performance while maintaining radio frequency performance.